The pros and cons of wood chip for biomass boilers
Posted by Laurence Jones on 23 May 2011 at 9:20 am
Renewable energy enthusiasts looking at biomass boilers have one big decision: wood chip or wood pellet? This first of a two part series concentrates on the former fuel type.
What is wood chip?
Wood chips come from three sources; roundwood (virgin timber cut to length), slab wood (off cuts from timber processing yards) and arboricultural arisings (the by-product of individual tree maintenance). The timber will usually be seasoned to reduce the moisture content to around 30% and then chipped using specific machinery that delivers a consistent size. The chips will then be moved to a store where they await transportation. They will be regularly monitored for moisture content.
Wood chips are transported to the site of the biomass boiler where they are fed into the appliance typically using an auger system (a device which feeds wood chips into the appliance, often from a dedicated store room).
Why choose wood chips?
Wood chips are usually available at a much more accessible price point than wood pellets. However, this saving does come with a higher installation cost for boilers in comparison to a pellet system. Chip systems also require larger floor area and vehicle access to the chip store.
Wood chip is used extensively as the fuel source for automated heating systems such as industrial and commercial sized boilers. Wood chip usage is also growing in the domestic market, with consumers looking to replace old gas and oil boilers, yet still retain the reliability and low maintenance costs.
What makes a good wood chip?
Good wood chips will have been processed from seasoned wood and should contain a moisture content and particle size that matches the requirement of the boiler it is intended by used with. Low quality chips (high moisture content, inconsistent particle size) may damage the appliance and reduce its reliability considerably.
Typically, larger boilers (greater than 1MW) are designed to be more robust, with a higher tolerance to lower grade chips.
Wood chips should be processed to achieve the dimensions and moisture content that your boiler requires. This helps to ensure that the feeding mechanism operates reliably and the burner will fire effectively. It will also keep maintenance costs at a minimum and will extend the life of the installation.
Historically, the universal standard for wood chip quality was ÖNORM M7 133. This specified a particle size which manufacturers designed their appliances around (e.g. G30 & G50). A new European standard (EN14961-4) is due to supersede this, with an update on chip size for manufacturers to adhere to.
Consumers should check that fuel suppliers are producing wood chips to these standards or carry certification (such as the Solid Biomass Assurance Scheme) to verify this.
About the author: Laurence Jones was marketing support officer at HETAS
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